Method and apparatus for compensation of beam property drifts detected by measurement systems outside of an excimer laser

ABSTRACT

A lithography laser system for incorporating with a semiconductor processing system includes a discharge chamber filled with a laser gas including molecular fluorine and a buffer gas, multiple electrodes within the discharge chamber and connected with a discharge circuit for energizing the laser gas, a resonator including the discharge chamber for generating a laser beam, and a processor. The processor runs an energy control algorithm and sends a signal to the discharge circuit based on said algorithm to apply electrical pulses to the electrodes so that the laser beam exiting the laser system has a specified first energy distribution over a group of pulses. The energy control algorithm is based upon a second energy distribution previously determined of a substantially same pattern of pulses as the group of pulses having the first energy distribution. The second energy distribution is determined for the laser beam at a location after passing the beam through beam shaping optical elements of the semiconductor processing system while a value of the energy of the laser beam exiting the laser system is maintained at an approximately constant first energy.

PRIORITY

This application claims the benefit of priority to U.S. provisionalapplications No. 60/189,729, filed Mar. 16, 2000, and No. 60/223,070,filed Aug. 4, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an energy stabilization method for alithography laser, and particularly to a method for compensating energytransients at a workpiece in a laser output energy control algorithm.

2. Discussion of the Related Art

Excimer lasers are typically used for industrial applications incombination with processing systems. Such systems can be lithographywafer scanners or TFT-annealing-systems for example. Typically theselaser systems have internal detectors to measure certain laserparameters. These laser parameters can include pulse energy, pulseenergy dose over a certain number of pulses, spatial beam profile in aplane of the processing system, temporal pulse duration or one or moreadditional laser beam parameters.

The excimer laser system used for those applications has normally onboard metrology measurement tools which allow the measurement andstabilization of similar parameters of the laser beam as pulse energy,pulse energy dose and so on. Under real conditions the detectors used inthe on board metrology and the detectors used in the processing tool maymeasure and/or exhibit different characteristics. For example, detectorsin the processing tool may be polarization sensitive, while detectors inthe laser system may be not sensitive to the polarization of theincident light.

In addition, the path of the beam between the laser beam detector andthe detector of the processing tool may include such optics as anaperture, such that an intensity difference between the detectors mayvary with the beam profile or beam width. Such variance may occur as aresult of a beam divergence varying over time such as may depend on thestructure of the burst sequencing. For example, a beam divergence may begreater during or after prolonged burst periods as compared to periodsof reduced exposure due to heating of optics or changes in the gasmixture temperature and/or composition.

SUMMARY OF THE INVENTION

In view of the above, a lithography laser system for incorporating witha semiconductor processing system is provided including a dischargechamber filled with a laser gas including molecular fluorine and abuffer gas, multiple electrodes within the discharge chamber andconnected with a discharge circuit for energizing the laser gas, aresonator including the discharge chamber for generating a laser beam,and a processor. The processor runs an energy control algorithm andsends a signal to the discharge circuit based on said algorithm to applyelectrical pulses to the electrodes so that the laser beam exiting thelaser system has a specified first energy distribution over a group ofpulses. The energy control algorithm is based upon a second energydistribution previously determined of a substantially same pattern ofpulses as the group of pulses having the first energy distribution. Thesecond energy distribution is determined for the laser beam at alocation after passing the beam through beam shaping optical elements ofthe semiconductor processing system while a value of the energy of thelaser beam exiting the laser system is maintained at an approximatelyconstant first energy.

In further view of the above, the laser beam energy exiting the laser isdetermined to change according to the second energy distribution as aresult of passing through said beam shaping optical elements. The beamexiting the laser has the substantially constant first energy over thegroup of pulses being transformed to the beam after the beam shapingoptical elements having the second energy distribution over the group ofpulses. The first energy distribution may be determined substantially asthe approximately constant first energy minus the second energydistribution, within a steady-state linear energy reduction multiple ofthe second energy distribution between the beam exiting the laser andthe beam after the beam shaping optics.

The first energy distribution may have the form:

E_(laser)(t)=E₀−KF(t), where K is a constant, F(t) is a function of timeand the second energy distribution has a form E(t)=E₁+F(t), where E₁ isa desired energy of the beam after the beam shaping optics, and E₀ isthe first energy of said beam exiting the laser, which first energy E₀is sufficient to produce the desired energy E₁ after the beam shapingoptics when the laser is operating in steady state. The constant K maybe E₁/E₀. The function F(t) may be Ae_(−(t/τ)), wherein A is a magnitudeof a transient overshoot, t is a time and τ is a time constant. Inaddition, the first energy distribution may be used in the energycontrol algorithm for a predetermined time after a long burst pause,after which the laser beam exiting said laser is maintained at thesubstantially constant first energy.

In further view of the above, a method for stabilizing a laser beamenergy at a location after beam shaping optical elements of asemiconductor fabrication system is provided including generating alaser beam and passing the beam through the beam shaping opticalelements. A first energy distribution is determined of the laser beam ata location after passing through the beam shaping optical elements overa burst pattern including a group of laser pulses while the laser beamis maintained at an approximately constant first energy. The processoris programmed with an energy control algorithm based on the first energydistribution.

Electrical pulses are applied to discharge electrodes of the lasersystem based on the energy control algorithm such that the laser beamexiting the laser system has a second energy distribution over the burstpattern including the group of pulses, such that an energy of the beamat the location after the beam shaping optics is controlled to besubstantially a desired constant second energy.

The laser beam energy exiting the laser may be determined to changeaccording to the second energy distribution as a result of passingthrough the beam shaping optical elements. The beam exiting the lasermay have the substantially constant first energy over the group ofpulses and be transformed to the beam after the beam shaping opticalelements and have the second energy distribution over the group ofpulses.

The applying step may include determining the first energy distributionsubstantially as the approximately constant first energy minus thesecond energy distribution, within a steady-state linear energyreduction multiple of the second energy distribution between the beamexiting the laser and the beam after the beam shaping optics.

The second energy distribution may have the form:

E_(laser)(t)=E₀−KF(t), wherein K is an constant, F(t) is a function oftime, and the second energy distribution has a form E(t)=E₁+F(t),wherein E₁ is the second energy and E₀ is the first energy, which firstenergy E₀ is sufficient to produce the desired energy E₁ after the beamshaping optics when the laser is operating in steady state. F(t) may beAe^(−(t/τ)), wherein A is a magnitude of a transient overshoot, t is atime and τ is a time constant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a laser system configuration according to apreferred embodiment for receiving information relating to beamtransformation between exiting the laser resonator and after passingthrough optical elements of a wafer processing tool.

FIG. 2 shows a qualitative, exemplary plot of a signal measured by aninternal energy stabilization loop detector of the laser system of FIG.1.

FIG. 3 shows a qualitative, exemplary plot of a signal measured by adetector positioned after optical elements of a wafer processing tool.

FIG. 4 schematically shows a laser system configuration according to apreferred embodiment for producing an output beam having a substantiallyconstant energy at a position after passing through optical elements ofa wafer processing tool.

FIG. 5 shows a plot of a function determined from data measuredaccording to the laser system configuration of FIG. 1 and used with anenergy control algorithm according to a preferred embodiment forcontrolling a laser beam output energy to stabilize an energy of thelaser beam at a position after passing through optical elements of awafer processing tool.

INCORPORATION BY REFERENCE

What follows is a cite list of references each of which is, in additionto those references cited above and below herein, including that whichis described as background, and the above invention summary, are herebyincorporated by reference into the detailed description of the preferredembodiment below, as disclosing alternative embodiments of elements orfeatures of the preferred embodiments not otherwise set forth in detailbelow. A single one or a combination of two or more of these referencesmay be consulted to obtain a variation of the preferred embodimentsdescribed in the detailed description below. Further patent, patentapplication and non-patent references are cited in the writtendescription and are also incorporated by reference into the detaileddescription of the preferred embodiment with the same effect as justdescribed with respect to the following references:

U.S. Pat. Nos. 5,710,787, 5,463,650, 6,008,497, 5,657,334, 6,005,879,4,611,270, 3,806,829, 6,141,081, 6,084,897, 4,997,573, 5,097,291,5,140,600 and 4,674,099; and

U.S. patent application Ser. Nos. 09/498,121, 09/688,561, and09/780,120, U.S. published application No. 20020012374, and U.S. Pat.Nos. 6,212,24, 6,490,307, 6,243,406, 6,243,405, and 6,389,052, each ofwhich is assigned to the same assignee as the present application.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically shows a lithography laser system such as a KrF, ArFor F₂ laser system in accord with a preferred embodiment, except that anenergy control algorithm is set for constant laser beam energy for thebeam exiting the laser rather than after the beam shaping optics 24, asis preferred and described in more detail below with respect to FIG. 4.In general, the system includes a laser chamber 2 filled with a gasmixture and having a pair of main electrodes 3 and one or morepreionization electrodes (not shown). The electrodes 3 are connected toa solid-state pulser module 4. A gas handling module 6 is connected tothe laser chamber 2. A high voltage power supply 8 is connected to thepulser module 4. A laser resonator is shown including the dischargechamber 2, a rear optics module 10 and a front optics module 12. Anoptics control module 14 communicates with the rear and front opticsmodules 10, 12. A computer or processor 16 controls various aspects ofthe laser system. A diagnostic module 18 receives a portion of theoutput beam 20 from a beam splitter 22.

The gas mixture in the laser chamber 2 typically includes about 0.1%F₂,1.0%Kr and 98.9% buffer gas for a KrF laser, 0.1%F₂, 1.0%Ar and 98.9%buffer gas for an ArF laser, and 0.1% F₂ and 99.9% buffer gas for the F₂laser. The buffer gas preferably comprises neon for the KrF laser, neonand or helium for the ArF laser, and helium and/or neon for the F₂ laser(see U.S. Pat. No. 6,157,662, which is hereby incorporated byreference). A trace amount of a gas additive such as xenon, argon orkrypton may be included (see U.S. patent application Ser. No.09/513,025, which is assigned to the same assignee as the presentapplication and is hereby incorporated by reference).

The gas mixture is preferably monitored and controlled using an expertsystem (see U.S. patent application Ser. No. 09/379,034, which isassigned to the same assignee as the present application and is herebyincorporated by reference). One or more beam parameters indicative ofthe fluorine concentration in the gas mixture, which is subject todepletion, may be monitored, and the gas supply replenished accordingly(see U.S. patent application Ser. Nos. 09/418,052 and 09/484,818, whichare assigned to the same assignee and are hereby incorporated byreference). The diagnostic module 18 may include the appropriatemonitoring equipment or a detector may be positioned to receive a beamportion split off from within the laser resonator (see U.S. patentapplication Ser. No. 60/166,967, which is assigned to the same assigneeas the present application and is hereby incorporated by reference). Theprocessor 16 preferably receives information from the diagnostic module18 concerning the halogen concentration and initiates gas replenishmentaction such as micro-halogen injections, mini and partial gasreplacements, and pressure adjustments by communicating with the gashandling module 6 (see U.S. patent application Ser. Nos. 09/780,120,09/734,459 and 09/447,882, which are assigned to the same assignee asthe present application and are hereby incorporated by reference).

Although not shown, the gas handling module 6 has a series of valvesconnected to gas containers external to the laser system. The gashandling module 6 may also include an internal gas supply such as ahalogen and/or xenon supply or generator (see the '025 application). Agas compartment or (not shown) may be included in the gas handlingmodule 6 for precise control of the micro halogen injections (see the'882 application and U.S. Pat. No. 5,396,514, which is herebyincorporated by reference).

The wavelength and bandwidth of the output beam 20 are also preferablymonitored and controlled. A preferred wavelength calibration apparatusesand procedures are described at U.S. Pat. Nos. 6,160,832, 4,905,243 andU.S. patent application Ser. No. 60/202,564, which are assigned to thesame assignee and are hereby incorporated by reference. The monitoringequipment may be included in the diagnostic module 18 or the system maybe configured to outcouple a beam portion elsewhere such as from therear optics module, since only a small intensity beam portion istypically used for wavelength calibration. The diagnostic module 18 maybe integrated with the front optics module 12, and the line-narrowingcomponents of the resonator may be integrated in the front optics module12, as well, such that only a HR mirror and an optional aperture areincluded in the rear optics module 10.

Preferred main electrodes 3 are described at U.S. Pat. No. 6,466,599,which is each assigned to the same assignee as the present applicationand is hereby incorporated by reference. Other electrode configurationsare set forth at U.S. Pat. Nos. 5,729,565 and 4,860,300, each of whichis hereby incorporated by reference. Preferred preionization units areset forth at U.S. patent application Ser. Nos. 09/247,887 and09/692,265, each of which is assigned to the same assignee as thepresent application and is hereby incorporated by reference. Thepreferred solid state pulser module 4 and the high voltage power supply8 are set forth at U.S. Pat. Nos. 6,020,723, 6,005,880, 6,226,307 and6,198,761, each of which is assigned to the same assignee as the presentapplication and is hereby incorporated by reference into the presentapplication.

The resonator includes optics for line-selection and also preferably fornarrowing the selected line (see U.S. Pat. Nos. 6,345,065, 6,154,470,6,426,966, 6,381,256, 6,285,701, 6,393,037, and 6,061,382 and U.S.patent application Nos. 60/212,193, 60/170,342, 60/166,967, 60/170,919,09/584,420,60/212,257, 60/212,301, 60/215,933, 60/124,241, 60/140,532,60/140,531, and 60/147,219, each of which is assigned to the sameassignee as the present application, and U.S. Pat. Nos. 5,761,236 and5,946,337, each of which is assigned to the same assignee as the presentapplication, and U.S. Pat. Nos. 5,095,492, 5,684,822, 5,835,520,5,852,627, 5,856,991, 5,898,725, 5,901,163, 5,917,849, 5,970,082,5,404,366, 4,975,919, 5,142,543, 5,596,596, 5,802,094, 4,856,018, and4,829,536, all of which are hereby incorporated by reference). Some ofthe line selection and/or line narrowing techniques set forth in thesepatents and patent applications may be used in combination with oralternative to any of the embodiments set forth herein.

Also, and particularly for the F2 and ArF laser systems, an enclosure(not shown) may seal the beam path of the beam 20 such as to keep thebeam path free of photoabsorbing species. Smaller enclosures may sealthe beam path between the chamber 2 and the optics modules 10 and 12.Preferred enclosures are described in detail in U.S. Pat. Nos.6,219,368, 6,442,182, 6,327,290, 6,399,916 and U.S. publishedapplication no. 20010028664, each of which is assigned to the sameassignee and is hereby incorporated by reference, and alternativeconfiguration are set forth at U.S. Pat. Nos. 5,559,584, 5,221,823,5,763,855, 5,811,753 and 4,616,908, all of which are hereby incorporatedby reference.

The beam 20 exiting the laser system is shown as being stabilized atconstant output energy. When the laser system is used in a waferfabrication system for photolithography, the beam traverses beam-shapingoptics 24 that form the beam. The beam-shaping optics 24 typicallyinclude an aperture and other optical elements. It is recognized in thepresent invention that the beam 20 having constant energy may betransformed as it passes through the beam shaping optics 24, such thatthe beam 25 after the beam shaping optics no longer exhibits a constantenergy. For example, an aperture of the beam shaping optics 24 may clipoff a varying amount of energy as the divergence of the beam 20 changeswith a varying degree of heating of the optics of the laser resonator.

The processor controls the laser to output laser pulses in bursts ofmany pulses followed by short pauses corresponding to the processing ofa semiconductor chip followed by the stepping of the wafer fabricationtool to a different chip. Several of these bursts followed by shortpauses may be generated. At certain points, long pauses wherein nobursts are generated occur while, e.g., a chip on a new sheet or waferis positioned or another reason. Thus, multiple series of bursts withshort pauses in between are followed by long pauses. During these longpauses, optics of the laser system may tend to cool down. The divergenceof the laser beam exiting the laser system varies with the heating ofthe optics, and thus the divergence may change dramatically during longburst pauses, when the optics cool down, and during the series ofbursts, when the optics warm back up. This produces a transient as theamount of energy transmitted through the beam shaping optics varies withthe divergence. It is, however, the energy of the beam after the beamshaping optics that is desired to remain constant, notwithstandingwhether the energy of the beam exiting the laser system varies.

FIG. 2 illustrates the energy of the laser beam as it exits the lasersystem when the energy stabilizes around a substantially constantenergy. For example, an internal detector of the laser system, such asof the diagnostic module 18, may be used for stabilization in a feedbackloop with the processor 16 and the power supply 8, after switching onthe laser under typical operation conditions. The qualitative plot showsof FIG. 2 shows an output energy of the laser system operating in burstmode, such as has been described above (see also U.S. Pat. Nos.5,710,787, 5,463,650, 6,008,497, 5,657,334, 6,005,879, and 5,140,600,and U.S. patent application Ser. Nos. 09/498,121, 09/379,034,09/447,882, 09/418,052, 09/484,818, 09/734,459, 60/178,620, 60/182,083and 60/186,011, each application being assigned to the same assignee asthe present application, and all of the above patents and patentapplications being hereby incorporated by reference into the presentapplication).

FIG. 3 shows the signal after switching on the laser with the same burstsequence as in FIG. 2 measured by a detector in a plane of theprocessing tool, such as the detector 26 of FIG. 1. It can be seen fromthe example that the magnitude of the signal measured by the externaldetector varies over time notwithstanding the fact that the laser hasits own stabilization loop. In the example the signal decreases to reacha steady state after a certain time.

The decrease of the signal may be caused by the explanation providedabove, the detector of the processing tool, the optical transfer findingof the processing tool, as well as all the optical means in front of thedetector. For example, the signal decrease may be caused by changes inthe optical characteristics by illumination with the excimer radiationof the components of the processing tool. On the other hand, the decaycould be caused by changes of the excimer laser internal optics andreflect a change of the signal which cannot be seen by the internaldetector. Such a change could for example be polarization. Similarbehavior one would observe if the external detector is polarizationsensitive while the internal detector is not sensitive for polarization.As it can be seen by a comparison of FIGS. 2 and 3, after having thelaser off for a certain time the relation between the internal signaland the external signal are reproducible (compare the pulses prior tothe pause labeled “a” with those after the pause labeled “b”,particularly in FIG. 3).

Now, this reproducible relationship is advantageously taken into accountin a software algorithm which compensates for the drift measured by theexternal detector. The result is that the external detector would see asignal that is not varying over time (and would look something like FIG.2) with the decay described above and shown at FIG. 3.

FIG. 4 illustrates a laser system according to a preferred embodimentwherein the processor is running a software algorithm that programs afunction into the internal laser system energy feedback loop, such thatthe laser beam 120 exiting the laser now has a non-constant energy. Thelaser beam 125 after the beam shaping optics, however, does have aconstant energy.

A function that describes the reduction of the energy of the beam 25after the beam shaping optics when the laser is configured according toFIG. 1 without the algorithm described with respect to FIG. 4 above, maybe an exponential function, for example. In this case the function maylook something like:

E(t)=Ae^(−(t/τ))+E₁, where τ is the time constant of the exponentialdecay, A is the amplitude of the overshoot which decays according to theexponential function, and E₁ is the desired energy of the beam after thebeam shaping optics when the energy has stabilized.

In general, the function would be E(t)=F(t)+E₁. When the energy at thedetector 26 of FIG. 1 is E(t), the energy of the beam 20 would besubstantially constant around E₀, as explained above. The algorithm ofthe preferred embodiment, then is advantageously altered to allow theenergy of the beam 120 exiting the laser of FIG. 4 to vary with time inorder to have a constant energy for the beam 125 after the beam shapingoptics 24 around a desired energy E₁, e.g., as follows:

E ₁ =E(t)−F(t)

The energy at the laser is accordingly programmed to be:

E _(laser) =E ₀ −E ₁ F(t)/E₀

In the example where F(t)=Ae^(−(t/τ)), then the energy at the laser isprogrammed to be:

E_(laser)=E₀−(E₁/E₀) Ae^(−(t/τ))=E₀−Ke^(−(t/τ)); where K is an offsetconstant. It is noted here that other functions may be used to describethe energy distribution after the beam shaping optics 24, such aspolynomial distributions, other forms of exponential distributions,e.g., that cutoff after a certain time, such as when the system enters asteady state, or the distribution may be measured and averaged, orotherwise.

FIG. 5 shows an energy versus time graph plotted based on data measuredat an internal detector (i.e., to the laser system), when the processoris programmed with the algorithm just described. The energy measured isbeing controlled in a feedback relationship using the processor 16. Theenergy of the laser beam is desired to be constant at a workpiece andnot necessarily as it exits the laser, such as is measured at theinternal detector of the diagnostic module 18 of the laser system. Theenergy is maintained from the beginning of a burst sequence until sometime (e.g., 200 seconds) afterwards according to the burst sequenceovershoot algorithm of the preferred embodiment, such that the energy isnot constant at the internal detector, as shown, until the steady stateis reached (e.g., 200 seconds following a long burst pause). The energyis, however, made to be substantially constant at a workpiece or afterthe beam shaping optics, as desired, based on the algorithm. Themeasured energy shown in FIG. 5 was determined to be that energy at theinternal detector for a beam such that the beam energy would be constantover the duration shown plotted in FIG. 5 at the external workpiece orafter the beam shaping optics 24.

While exemplary drawings and specific embodiments of the presentinvention have been described and illustrated, it is to be understoodthat that the scope of the present invention is not to be limited to theparticular embodiments discussed. Thus, the embodiments shall beregarded as illustrative rather than restrictive, and it should beunderstood that variations may be made in those embodiments by workersskilled in the arts without departing from the scope of the presentinvention as set forth in the claims that follow, and equivalentsthereof.

In addition, in the method claims that follow, the operations have beenordered in selected typographical sequences. However, the sequences havebeen selected and so ordered for typographical convenience and are notintended to imply any particular order for performing the operations,except for those claims wherein a particular ordering of steps isexpressly set forth or understood by one of ordinary skill in the art asbeing necessary.

What is claimed is:
 1. A lithography laser system for incorporating witha semiconductor processing system that includes beam shaping opticalelements, comprising: a discharge chamber filled with a laser gasincluding molecular fluorine and a buffer gas; a plurality of electrodeswithin the discharge chamber and connected with a discharge circuit forenergizing the laser gas; a resonator including the discharge chamberfor generating a laser beam, wherein the laser beam exits the lasersystem and passes through the beam shaping optical elements of thesemiconductor processing system; and a processor for running an energycontrol algorithm and sending a signal to the discharge circuit based onsaid algorithm to apply electrical pulses to the electrodes so that thelaser beam exiting the laser system and before entering the beam shapingoptical elements has a specified first energy distribution over aplurality of pulses that results in a substantially constant energy ofthe laser beam exiting the beam shaping optical elements, and whereinsaid energy control algorithm is based upon a second energy distributionof the laser beam previously determined of a substantially same patternof pulses as said plurality of pulses having said first energydistribution, said second energy distribution being determined after thelaser beam passes through the beam shaping optical elements while theenergy of the laser beam exiting the laser system and entering the beamshaping optical elements is maintained at an approximately constantfirst energy.
 2. The system of claim 1, wherein said first energydistribution has a form: E_(laser)(t)=E₀−KF(t), where K is a constant,F(t) is a function of time and said second energy distribution has aform E(t)=E₁+F(t), and wherein E₁ is a desired energy of said beam aftersaid beam shaping optical elements, and E₀ is said first energy of saidbeam exiting said laser, which first energy E₀ is sufficient to producethe desired energy E₁ after the beam shaping optical elements when thelaser is operating in steady state.
 3. The system of claim 2, whereinF(t)=Ae^(−(t/τ)), wherein A is a magnitude of a transient overshoot, tis a time and τ is a time constant.
 4. The system of claim 3, whereinsaid first energy distribution is used in the energy control algorithmfor a predetermined time after a long burst pause, after which saidlaser beam exiting said laser is maintained at said substantiallyconstant first energy.
 5. The system of claim 1, wherein said firstenergy distribution is used in the energy control algorithm for apredetermined time after a long burst pause, after which said laser beamexiting said laser is maintained at said substantially constant firstenergy.
 6. A lithography laser system for use with a semiconductorprocessing system, wherein the semiconductor processing system includesbeam shaping optical elements separate from and downstream of the lasersystem, said laser system comprising: a discharge chamber filled with alaser gas including molecular fluorine and a buffer gas; a plurality ofelectrodes within the discharge chamber and connected to a dischargecircuit for energizing the laser gas; a resonator including thedischarge chamber for generating a laser beam, wherein the laser beamexits the laser system and passes through the beam shaping opticalelements of the semiconductor processing system; and control means forsending a signal to the discharge circuit to apply electrical pulses tothe electrodes to control the time distribution of output energy of thelaser beam so that the energy of a burst of laser pulses exiting thebeam shaping optical elements is substantially constant over time. 7.The system of claim 6, wherein the control means includes means fordetermining how the beam shaping optical elements affects the timedistribution of energy of a burst of laser pulses having constant energyoutput from the laser system and input to the beam shaping opticalelements, and for using that determination to compensate for the effectof the beam shaping optical elements by varying the signal to thedischarge circuit.
 8. The system of claim 6, wherein the control meansincludes means for determining a time distribution of energy of theburst of laser pulses entering and exiting the beam shaping opticalelements.
 9. The system of claim 8, wherein the control means includesmeans for determining the effect of the beam shaping optical elements onthe laser beam by comparing the time distribution of energy of the burstof laser pulses entering and exiting the beam shaping optical elements,and wherein the control means compensates for the effect of the beamshaping optical elements by varying the signal to the discharge circuitto adjust the time distribution of energy of the laser beam entering thebeam shaping optical elements so that the energy of the laser beamexiting the beam shaping optical elements is substantially constant overtime.
 10. The system of claim 6, wherein the control means controls thetime distribution of output energy of the laser beam for a predeterminedtime after a long burst pause, after which time the output energy of thelaser beam is substantially constant.
 11. A lithography laser system forincorporating with a semiconductor processing system that includes beamshaping optical elements, comprising: a discharge chamber filled with alaser gas including molecular fluorine and a buffer gas; a plurality ofelectrodes within the discharge chamber and connected to a dischargecircuit for energizing the laser gas; a resonator including thedischarge chamber for generating a laser beam, wherein the laser beamexits the laser system and passes through the beam shaping opticalelements of the semiconductor processing system; a diagnostic modulethat monitors the energy of the laser beam as it exits the resonator andbefore it enters the beam shaping optical elements; and a processorcoupled to the diagnostic module and the discharge circuit and includingmeans for sending a signal to the discharge circuit to control theenergy of the laser beam so that the laser beam exiting the beam shapingoptical elements has a substantially constant energy over a plurality ofpulses, and wherein said means for sending a signal uses a previouslydetermined time distribution of energy of the laser beam measured afterthe laser beam passes through the beam shaping optical elements whilethe energy of the laser beam monitored by the diagnostic module beforethe beam shaping optical elements is maintained at an approximatelyconstant energy.
 12. A lithography laser system for use with asemiconductor processing system that includes beam shaping opticalelements, said laser system comprising: a discharge chamber filled witha laser gas including molecular fluorine and a buffer gas; a pluralityof electrodes within the discharge chamber and connected to a dischargecircuit for energizing the laser gas; a resonator including thedischarge chamber for generating a laser beam, wherein the laser beamexits the laser system and passes through the beam shaping opticalelements of the semiconductor processing system; and a processor forrunning an energy control algorithm and sending a signal to thedischarge circuit based on said algorithm to apply electrical pulses tothe electrodes so that the laser beam exiting the beam shaping opticalelements has a substantially constant energy over a plurality of pulses,and wherein said energy control algorithm is based on an energydistribution of the laser beam previously determined of a substantiallysame pattern of pulses as said plurality of pulses, said energydistribution being determined after the laser beam passes through thebeam shaping optical elements while the energy of the laser beamentering the beam shaping optical elements is maintained at anapproximately constant energy.
 13. A method for stabilizing the energyof a laser beam at a location downstream of beam shaping opticalelements of a semiconductor fabrication system, wherein the beam shapingoptical elements are separate from and downstream of a laser system thatgenerates the laser beam, wherein the laser system includes laser gascontaining molecular fluorine and a buffer gas, and wherein the methodcomprises the steps of: generating a first series of laser pulses havingan approximately constant time distribution of energy upstream of thebeam shaping optical elements; determining how the beam shaping opticalelements affects the time distribution of energy of the laser pulses bymeasuring a time distribution of energy of the first series of laserpulses downstream of the beam shaping optical elements; and generating asecond series of laser pulses having a time distribution of energyupstream of the beam shaping optical elements that compensates for theeffect of the beam shaping optical elements on the energy distributionof the laser beam and results in a substantially constant timedistribution of energy of the second series of laser pulses downstreamof the beam shaping optical elements.
 14. A method for stabilizing theenergy of a laser beam at a location downstream of beam shaping opticalelements of a semiconductor fabrication system, wherein the beam shapingoptical elements are separate from and downstream of a laser system thatgenerates the laser beam, wherein the laser system includes laser gascontaining molecular fluorine and a buffer gas, and wherein the methodcomprises the steps of: generating a first series of laser pulses havingan approximately constant energy upstream of the beam shaping opticalelements; measuring a time distribution of energy of the first series oflaser pulses downstream of the beam shaping optical elements; andgenerating a second series of laser pulses having a time distribution ofenergy upstream of the beam shaping optical elements that results in asubstantially constant time distribution of energy of the second seriesof laser pulses downstream of the beam shaping optical elements.
 15. Amethod for stabilizing the energy of a laser beam at a locationdownstream of beam shaping optical elements of a semiconductorfabrication system, wherein the beam shaping optical elements areseparate from and downstream of a laser system that generates the laserbeam, wherein the laser system includes laser gas containing molecularfluorine and a buffer gas, and wherein the method comprises the stepsof: generating a first series of laser pulses having a substantiallyconstant energy upstream of the beam shaping optical elements; measuringa time distribution of energy of the first series of laser pulsesdownstream of the beam shaping optical elements; and controlling theenergy of the laser beam upstream of the beam shaping optical elementsbased on the measured energy of the first series of laser pulses so thatthe time distribution of energy of the laser beam downstream of the beamshaping optical elements is substantially constant.
 16. A method foroperating a lithography laser system for use with a semiconductorprocessing system, wherein the semiconductor processing system includesbeam shaping optical elements separate from and downstream of the lasersystem, wherein the laser system includes a laser gas that includesmolecular fluorine and a buffer gas and a controller that controls theoutput energy of the laser beam, said method comprising the steps of:controlling the time distribution of output energy of the laser beamupstream of the beam shaping optical elements so that the energy of aburst of laser pulses downstream of the beam shaping optical elements issubstantially constant over time.
 17. The system of claim 16, whereinthe step of controlling includes determining how the beam shapingoptical elements affects the time distribution of energy of a burst oflaser pulses having constant energy output from the laser system andinput to the beam shaping optical elements, and for using thatdetermination to compensate for the effect of the beam shaping opticalelements.
 18. The system of claim 16, wherein the step of controllingincludes determining a time distribution of energy of the burst of laserpulses entering and exiting the beam shaping optical elements.
 19. Thesystem of claim 18, wherein the step of controlling includes determiningthe effect of the beam shaping optical elements on the laser beam bycomparing the time distribution of energy of the burst of laser pulsesentering and exiting the beam shaping optical elements, and compensatingfor the effect of the beam shaping optical elements by adjusting thetime distribution of energy of the laser beam entering the beam shapingoptical elements so that the energy of the laser beam exiting the beamshaping optical elements is substantially constant over time.
 20. Thesystem of claim 16, wherein the step of controlling occurs for apredetermined time after a long burst pause, after which time the outputenergy of the laser beam is substantially constant.